1. Field of the Invention
This invention concerns a dielectric-barrier discharge lamp light source and power supply that incorporates what is called a dielectric-barrier discharge lamp, a type of discharge lamp that uses light radiated by excimer molecules formed by dielectric-barrier discharge.
2. Description of the Related Art
Technology related to this invention is found in, for example, U.S. Pat. No. 4,983,881 (Japanese Kokai Patent document H2-7353) which deals with a dielectric-barrier discharge lamp. This document describes a radiator in which a discharge vessel is filled with a discharge gas that can form excimer molecules, the excimer molecules being formed by means of dielectric-barrier discharge, and light being radiated from the excimer molecules.
The dielectric-barrier discharge lamp described above and a light source that incorporates such a lamp has a wide range of potential applications because of a number of advantages not found in conventional low-voltage mercury discharge lamps or high-voltage arc discharge lamps. In particular, with the growing interest in the problem of environmental pollution in recent years, one of the most important applications is non-polluting treatment of materials by means of a photochemical reaction using ultraviolet light. There is, accordingly, extremely great interest in increasing the power of dielectric-barrier discharge lamp light sources.
However, there have been a number of major problems that this conventional technology has been unable to resolve. The first of the problems is the necessity of improving the light efficiency of lamps in order to meet the high interest in reduced power consumption, reduced generation of heat by the lamp, and improved lamp longevity.
The second problem is the need to popularize the use of superior ultraviolet radiation technology by making it more economical.
These issues are becoming increasingly important as the light sources increase in output, and thus, power consumption.
The conditions for improvement of the light efficiency of lamps are explained below. The dielectric-barrier discharge lamp (2) has one or two dielectrics (6, 7) sandwiching a discharge plasma space (3) between electrodes (4, 5). FIG. 1 shows a dielectric-barrier discharge lamp with two dielectrics (6, 7).
When the dielectric-barrier discharge lamp (2) is turned on, a high-frequency, alternating-current voltage of, for example, 10 kHz to 200 kHz and 2 kV to 10 kV from the power supply is impressed on the two electrodes (4, 5). Because the dielectrics (6, 7) intervene between the discharge plasma space (3) and the electrodes, current does not flow directly from the electrodes (4, 5) to the discharge plasma space (3), and the dielectrics (6, 7) exhibit a condenser effect with respect to the flow of current. That is, equivalent charges of opposite sign to those between the surfaces of the electrodes (4, 5) are induced by the polarization of the dielectric, and the discharge occurs between the surfaces of the dielectrics (6, 7) that face across the discharge plasma space (3).
Because little current flows along the surfaces of the dielectrics (6, 7) that face across the discharge plasma space (3), the charge that is induced on the surfaces of the dielectrics (6, 7) that face across the discharge plasma space (3) in the area where the discharge occurred is neutralized by the charge shifted by the discharge, and the electric field in the discharge plasma space (3) is reduced, so that the discharge current stops even if the voltage impressed on the electrodes (4, 5) continues. However, the discharge current is sustained if the voltage impressed on the electrodes (4, 5) is further increased. After one discharge, there is no further discharge in the area where the discharge occurred until the polarity of the voltage impressed on the electrodes (4, 5) is reversed.
In the case of a dielectric-barrier discharge lamp filled with xenon gas, for example, the xenon gas is separated by the discharge into ions and electrons, to form xenon plasma. Within this plasma, the xenon that is excited to a certain energy level bonds, and excimer molecules are formed. Xenon excimers decay after the passage of a given lifespan, and the energy released at that time is emitted as photons with a vacuum ultraviolet wavelength. In order for dielectric-barrier discharge lamps to function efficiently as vacuum ultraviolet light sources, it is necessary to form these excimer molecules efficiently.
The major factor preventing the efficient formation of excimer molecules is the excitation of the discharge plasma to energy levels that do not contribute to the formation of excimer molecules.
The electron movement of discharge plasma before and after discharge commences is collective, and energy is high but the temperature is low. In this state, there is a high probability that the discharge plasma will transition to the resonant state necessary to formation of excimer molecules. As the discharge time lengthens, however, the electron movement of the plasma gradually becomes heated; that is, it reaches a heat equilibrium known as the Maxwell-Boltzmann distribution. Then, the plasma temperature increases and there is a greater probability of transition to a state of high excitation where excimer molecules cannot form.
Moreover, if excimer molecules do form, subsequent discharge sometimes breaks down the excimer molecules before they naturally decay and emit the desired photons after the passage of their lifespan. Actually, in the case of xenon excimers, a period of about 1 xcexcs is needed from the beginning of discharge until a vacuum ultraviolet wavelength photon is emitted; subsequent discharge or re-discharge within that period reduces the efficiency of excimer light generation.
In other words, it is known to be most important to keep the subsequent discharge energy as low as possible once discharge has begun.
Even if the discharge time is short, the probability of transition to the same high-excitation state will increase if too much energy is injected during the discharge period. Plasma that has transitioned to a high-excitation state alleviates that state by emitting infrared radiation and just raises the temperature of the lamp without contributing to excimer light generation.
That is, it is necessary to drive the discharge so as to suppress the excitation of discharge plasma to an energy level that does not contribute to the formation of excimer molecules.
Japanese kokai patent document H1-243363 proposes achievement of high-efficiency excimer light generation by any means of pulse discharge, including dielectric-barrier discharge. This proposal follows the condition that, once discharge has begun, the energy of the subsequent discharge is kept as low as possible. However, the description of this proposal deals with which parameters to adjust to make excimer light generation efficient; it gives no concrete indication of the effective conditions of the parameter values or of how to constitute a power supply that can realize those conditions.
Among the lamp voltage waveforms that have the potential to satisfy the conditions for discharge that will suppress the excitation of discharge plasma to an energy level that does not contribute to the formation of excimer molecules, as described above, one of the simplest candidates is thought to be a short waveform of optimal amplitude. There are, in fact, improvement proposals, for drive waveforms for fluorescent lamps using dielectric-barrier discharge, such as Japanese kokai patent document H6-163006. That states that the brightness of fluorescent lamps is increased by driving them with a stream of short with positive polarity, or with alternating current with a short waveform. Regarding the frequency and duty cycle of the stream of short pulses or the short-waveform current, it records experimental results on changes of brightness relative to changes in the voltage impressed, and explains that efficiency is improved over the conventional sine-wave drive.
However, this sort of simple waveform has a major problem when it comes to actually putting together a power supply. This problem arises from the fact that there is an impulsive flow of current only at the instant of the rise or fall of the voltage impressed on the lamp because, as stated above, the dielectric (6, 7) of the dielectric-barrier discharge lamp is a structure in which current flows because it acts as a condenser and basically is a condenser. This situation is illustrated in FIG. 3, a typical representation of the lamp voltage waveform (Vs(t)) and the lamp current waveform (Is(t)).
Normally, power supplies that generate a high-voltage alternating current to impress on dielectric-barrier discharge lamps use inverter circuits and step-up transformers, in which the current that flows through the primary winding increases in proportion to the ratio by which the voltage is stepped up from the primary side to the secondary side. For example, if the peak current value for the lamp current waveform is 3 A and the step-up transformer has a step-up ratio of 20, the peak current value flowing through the primary winding will actually reach 60 A.
That current value is not impossible to achieve, but an inverter circuit switch element that can withstand that current will be expensive, making it difficult to resolve the second problem mentioned above. This peak current value will of course vary, depending on the rating of the dielectric-barrier discharge lamp to be lit and the structure of the step-up transformer, but in any case, it is necessary to make that value a small one. The proposal in Japanese kokai patent document H6-163006, mentioned above, does not state specific guidelines for resolving this problem.
This invention provides a dielectric-barrier discharge lamp light source and power supply that simultaneously resolves both the first problem stated above, which is the problem of improving the lamp""s lighting efficiency to meet demands for reduced power consumption, reduced lamp heat and longer lamp life, and the second problem stated above, which is the problem of making the lamp more economical to popularize the use of superior ultraviolet light.
In order to solve the problems stated above, the dielectric-barrier discharge lamp light source of this invention has the following structure.
(1). A dielectric-barrier discharge lamp light source with a dielectric-barrier discharge lamp (2) comprising a discharge plasma space (3) filled with a discharge gas in which excimer molecules are formed by dielectric-barrier discharge, and two electrodes (4, 5) to induce a discharge phenomenon in the discharge gas, at least one of which is separated from the discharge gas by dielectrics (6, 7); and with a power supply to impress a roughly cyclical, high-voltage alternating current on the electrodes (4, 5) of the dielectric-barrier discharge lamp; such that when the power supply (1) completed one discharge in the dielectric-barrier discharge lamp (2) and the voltage impressed on the lamp changes toward the next discharge,
there is a period of gradual change in the lamp voltage waveform (Vs(t)) before reaching the voltage for the beginning of the next effective discharge (+Ei, xe2x88x92Ei), following which the voltage changes sharply.
(2). Also, a dielectric-barrier discharge lamp light source as described above, characterized by having a voltage VA when one discharge is completed and a voltage VB when the next discharge is completed with a voltage differential xcex94Vx between, as well as a voltage differential xcex94Vy between voltage VA and the voltage VF at the end of the period of gradual change of the voltage impressed on the lamp,
satisfying the condition 0.3 xcex1xcex94Vy/xcex94Vx xcex10.9.
(3). Also, a dielectric-barrier discharge lamp light source power supply for the dielectric-barrier discharge lamp (2), which has a discharge plasma space (3) filled with discharge gas in which excimer molecules are formed by dielectric-barrier discharge, and two electrodes (4, 5) to induce a discharge phenomenon in the discharge gas, at least one of which is separated from the discharge gas by dielectrics (6, 7), which power supply impresses a roughly cyclical, high-voltage alternating current on the electrodes (4, 5) of the lamp (2); the power supply comprising a chopper circuit (26) that outputs the voltage of a DC power supply (12) stepped up to a higher DC voltage, a step-up transformer (10) that produces a high-voltage alternating current on its secondary side, and an inverter circuit (13) to convert the output of the chopper circuit (26) to alternating current and feed it to the primary side of the step-up transformer (10); such that the gate signal (Gc) controlling the switch element of the chopper circuit (26) is generated in synchronization with the gate signals (GU, GL) controlling the switch elements of the inverter circuit (13).
(4). Also, a power supply (1) which is the dielectric-barrier discharge lamp light source power supply described above.